1. Field of the Invention
The present invention relates to a photovoltaic cell using a semiconductor junction.
2. Description of the Background Art
In recent years, photovoltaic cells having pn junctions between n-type single crystalline silicon substrates and p-type amorphous silicon films have been developed. In such photovoltaic cells, fill factors F.F. must be improved while maintaining high short-circuit currents Isc and open voltages Voc in order to improve photoelectric conversion efficiencies.
In the junctions between the n-type single crystalline silicon substrates and the p-type amorphous silicon films, however, a large number of interface states exist. Therefore, recombination of carries occurs, so that the open voltages Voc are reduced.
Therefore, a photovoltaic cell having an HIT (Heterojunction with Intrinsic Thin-Layer) structure in which a substantially intrinsic amorphous silicon film (an i-type amorphous silicon film) is inserted between an n-type single crystalline silicon substrate and a p-type amorphous silicon film in order to restrain recombination of carriers in a junction between the n-type single crystalline silicon substrate and the p-type amorphous silicon film has been proposed (see JP 2001-345463 A, for example).
Alternatively, a photovoltaic cell having a BSF (Back Surface Field) structure in which an i-type amorphous silicon film and an n-type amorphous silicon film are formed on a back surface of an n-type single crystalline silicon substrate in order to restrain recombination of carriers on the back surface of the n-type single crystalline silicon substrate has been also known.
In order to further enhance the photoelectric conversion efficiency of the photovoltaic cell, the area of the amorphous silicon film formed on the main surface and the back surface of the n-type single crystalline silicon substrate may be made as large as possible to improve the collection factor of photoproduction carriers. That is, the amorphous silicon film may be formed on the respective entire areas of the main surface and the back surface of the n-type single crystalline silicon substrate.
In the above-mentioned formation of the amorphous silicon film, a plasma CVD (Chemical Vapor Deposition) method has been generally used. Here, an attempt to form an i-type amorphous silicon film and a p-type amorphous silicon film by the plasma CVD method in the entire area of the main surface of the n-type single crystalline silicon substrate may cause the i-type and p-type amorphous silicon films to be detoured around a side surface and the back surface of the n-type single crystalline silicon substrate. Similarly, an attempt to form the i-type amorphous silicon film and the n-type amorphous silicon film by the plasma CVD method in the entire area of the back surface of the n-type single crystalline silicon substrate may cause the i-type and n-type amorphous silicon films to be detoured around the side surface and the main surface of the n-type single crystalline silicon substrate. In this case, the p-type amorphous silicon film on the main surface of the n-type single crystalline silicon substrate and the n-type amorphous silicon film on the back surface thereof come into contact with each other on the side surface of the n-type single crystalline silicon substrate, so that a leak current is generated. Thus, the photoelectric conversion efficiency of the photovoltaic cell is reduced.
In order to solve such a problem, therefore, a photovoltaic cell in which a p-type amorphous silicon film on a main surface of an n-type single crystalline silicon substrate and an n-type amorphous silicon film on a back surface thereof are prevented from coming into contact with each other by reducing the respective areas of an i-type amorphous silicon film and the n-type amorphous silicon film that are formed on the back surface of the n-type single crystalline silicon substrate has been proposed (see JP 2001-44461 A, for example).
In the photovoltaic cell in which the area of the amorphous silicon film formed on the back surface of the n-type single crystalline silicon substrate is reduced, as described above, however, generated photocarriers are not collected in a portion having no amorphous silicon film formed therein on the back surface of the n-type single crystalline silicon substrate, so that the photocarriers may be recombined at surface states. Particularly in a case where a surface, on the side of the n-type amorphous silicon film, of the photovoltaic cell is a light incidence surface, an output current of the photovoltaic cell is reduced.